Contributed by the Mechanisms and Robotics Committee of ASME for publication in the JOURNAL OF MECHANISMS AND ROBOTICS. Manuscript received April 22, 2018; final manuscript received August 30, 2018; published online October 5, 2018. Assoc. Editor: David J. Cappelleri.

Abstract

Elastic storage has been reported to help flying insects save inertial power when flapping their wings. This motivates recent research and development of elastic storage for flapping-wing micro air vehicles (fwMAVs) and their ground (tethered) flight tests. The previous designs of spring-loaded transmissions are relatively heavy or bulky; they have not yet been adopted by freely hovering prototypes of fwMAVs, especially those with four flapping wings. It is not clear if partial elastic storage can still help save power for flapping flight while not overloading the motorized transmission. Here, we developed ultralight and compact film hinges as elastic storage for four flapping wings. This spring-assisted transmission was motor driven such that the wing beat frequency was higher than the natural frequency of elastically hinged wings. Our experiments show that spring recoil helps accelerate wing closing thus generating more thrust. When powered by a 3.18 g brushless motor, this 13.4 g fwMAV prototype with spring-assisted transmission can take off by beating four flexible wings (of 240 mm span) with up to 21–22 g thrust generation at 22–23 Hz. Due to lower disk loading and high-speed reduction, indirect drive of the four elastically hinged wings can produce a thrust per unit of electrical power of up to 4.6 g/W. This electrical-power-specific thrust is comparable to that generated by direct drive of a propeller, which was recommended by the motor (AP-03 7000kv) manufacturer.

A prototype of spring-assisted motorized transmission for four flapping wings (X wings): (a) a complete assembly without a tail, (b) breakdown of component weights, (c)–(d) photographs of the wing transmission in angled and side views, (e)–(f) schematic drawings of the wing transmission in angled and side views, and (g)–(h) front views with the presence or absence of a brushless motor

Design and construction of a light wing plane: (a)–(b) schematic drawing showing the assembly of wing film and spar, (c) photograph of two overlapped wing planes with carbon spar reinforcement, (d) design and dimension of the wing film, and (e) photographs of the components

Static deformation and recoil of a polyimide film hinge that supports a wing pair: (a) static deformation under a deadweight, (b) a snapshot of recoil post the deadweight release, (c) the applied moments required to bend the film hinge, and (d) transient of recoil in terms of stroke angle

Wing kinematics of four wings on a 20 mm wide hinge pair: (a) snapshots showing the wing opening and closing at 13.Hz, (b)–(c) stroke angle and speed as measured from the midchord rib of one of the four wings, and (d)–(e) pitch angle and speed as measured from the mid-chord rib of one of the four wings

The effect of hinge stiffness on frequency-dependent wing kinematics: (a)–(b) amplitudes of stroke angle and speed at approximately 17 Hz, (c)–(d) amplitudes of pitch angle and speed at approximately 17 Hz, (e)–(f) frequency-dependent amplitudes of stroke and stroke speed, and (g)–(h) frequency-dependent amplitudes of pitch and pitch speed

Take-off of an 13.4 g MAV prototype with flapping X wings (on 20 mm film hinges), along a guided wire. See supplemental Movie S1 which is available under the “Supplemental Data” tab for this paper on the ASME Digital Collection.

Power components incurred by a brushless motor for driving a transmission with 20 mm wide hinged wings: (a) frequency dependence and (b) transient powers incurred for beating wings at 17.0 Hz for two cycles

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